The .MP3 file must be 128 kbps to be played in Re-Volt. If this is not your case, you can try a free converter called Format Factory to edit your audio file.

Once you have the file in proper conditions, you have to place it in the track folder (example: frontend, garden1, etc.) or in main revolt directory. Most people prefer to put the music inside each track's folder but both are Ok.

Then the most important thing is to edit the trackname.ini file as shown below:

thanks alot Phantom, that really helped. And I also realized that the mp3s I had even had .mp3 but I got removed .mp3 off the mp3 files themselves. That was actually the main problem of my mistake. Anyways, I'm now enjoying custom music in-game. Thanks again!